1. Field of the Invention
The present invention relates to an arrangement for synchronization of nodes in VDSL-systems, or more exactly, synchronization of optical VDSL-nodes which share a common part of a cable in the access network between the subscribers and the local station. The invention provides a time synchronization towards an external system, for instance GPS, which gives a time reference by which the different nodes can be synchronized. The synchronization reduces the near end cross talk between the VDSL-systems in the different nodes.
2. Discussion of the Background
Systems with VDSL (Very high Data rate Digital Subscriber Line) are previously known. In the VDSL-system a node, preferably optical, is placed between the copper cable to the subscriber and an optical fibre to the local station. By minimizing the length of the copper cable, the data rate can be maximized. In some cases different VDSL-systems in different optical nodes can share the same cable in a part of the access network. A problem is that near end cross talk between the systems can arise in the optical nodes if the optical nodes are not synchronized so that the orthogonality between the signals are maintained.
The invention solves the problem by synchronizing the nodes towards an external system which gives a stable time reference, for instance GPS. Synchronization towards external systems is as such previously known, for instance in the paging system Mini Call which utilizes GPS-receivers. This, however, solves another problem than the present invention.
Consequently the following invention provides an arrangement for synchronization of nodes in VDSL-systems which have VDSL-nodes between a local station and subscribers of above mentioned kind, where the nodes have separate connections to one point, and a common access connection from the point to the subscribers. According to the invention the nodes are synchronized to a common external time reference.
Preferably respective node includes a receiver for a synchronization signal and an internal oscillator with high stability for delivering a stable clock signal.
The invention is defined in the following patent claims which also indicate preferred embodiments of the invention.
Thanks to the arrangement of the invention, a minimization of the cross talk is made possible between VDSL-systems which results in higher capacity and longer range.